I'm not sure if I've ever heard of any dentist using a waterpik to clean a patient's teeth. If so, that'd be the first I heard of it.
Are you sure he wasn't using an ultrasonic scaler? Its a scaler that makes a screech-y sound and generates alot of water. The scaler tip vibrates quite frequenty, generating alot of heat, that is cooled off by the water. Perhaps that is what he was using.
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I highly doubt your dentist used a waterpik to scale your teeth. As Dr. Tsang suggested, it was most likely an ultrasonic scaler. Even with an ulatrasonic scaler though, your dentist should have gone back with regular instruments to make sure all the calculus was removed.
For a brief understanding of what else should go into a scaling appointment, I find this page on scaling to be helpful.